Electrochemical synthesis of perfluoroalkylfluorophosphoranes

ABSTRACT

The invention relates to a process for preparing perfluoroalkylfluorophosphoranes of the general formulawheren is 1, 2, 3, 4, 5, 6, 7 or 8m is +1 or -1 andy is 1, 2 or 3,where the ligands (CnF2n+m) may be identical or different, and also to their use as electrolytes and as precursors for conducting salts, and to their employment in lithium batteries.

The invention relates to a process for preparingperfluoroalkylfluorophosphoranes of the general formula

(C_(n)F_(2n+m))_(y)PF_(5−y)  (I)

where

n is 1, 2, 3, 4, 5, 6, 7 or 8

m is +1 or −1 and

y is 1, 2 or 3,

where the ligands (C_(n)F_(2n+m)) may be identical or different, andalso to their use as electrolytes and as precursors for conductingsalts, and to their employment in lithium batteries.

Perfluoroalkylfluorophosphoranes are of widespread interest as startingmaterials for synthesizing a variety of organofluorophosphorus compounds(N. V. Pavlenko et. al., Zh. Obshch. Khim. (Russ.) 1989, Vol. 59,534-537).

Perfluoroalkylfluorophosphoranes may be synthesized in a variety ofways, e.g. starting from elemental phosphorus and perfluoroalkyl iodides(F. W. Bennett et. al., J. Chem. Soc. 1953, 1565-1571). This reactionnormally leads initially to the formation of a complex mixture of mono-,bis- and trisperfluoroalkylphosphanes, which can then be converted bychlorination and fluorination processes into the correspondingphosphoranes (M. Görg et. al., J. Fluorine Chem. 1996, Vol. 79,103-104). A variety of by-products are produced by the side reactions,and these are difficult to remove and dispose of. One of thedisadvantages of this synthetic route is the reaction in the presence ofHg, which remains detectable in the downstream products. Productsprepared by this process are unsuitable for use in batteries. Inaddition, only small laboratory-scale batches can be prepared.

A relatively new method (J. J. Kampa et. al., Angew. Chem. 1995, Vol.107, 1334-1337) for synthesizing trisperfluoroalkyldifluorophosphoranesis to react elemental fluorine with the corresponding alkyl phosphanes.The disadvantages of this method are complicated operation and veryexpensive starting materials. The fluorinated solvents needed for theprocess are expensive to prepare, special safety precautions have to betaken when they are used, and they are expensive to dispose of onceused.

The most convenient method known hitherto is the electrochemicalfluorination of trialkylphosphine oxides described in DE 26 20 086,using Simons' electrochemical fluorination. The disadvantages of theprocess are that only trisperfluoroalkylphosphoranes can be prepared andthat the yields, from 40 to 50%, are low and decrease still further asthe chain length of the alkyl radical rises. Another disadvantage is theunavoidable parallel formation of toxic and explosive by-products, e.g.oxygen difluoride.

The methods known hitherto for obtaining perfluoroalkylphosphoranes byelectrochemical fluorination require the presence of stronglyelectro-negative substituents, such as fluorine or chlorine, or ofoxygen, to stabilise the electrofluorination starting materials withrespect to the operating medium (DE 19641138 and WO 98/15562). This isconfirmed in the literature (Journal of Fluorine Chemistry 75, 1995,157-161).

The object of the present invention is therefore to provide acost-effective process which is simple to carry out and which gives theperfluoroalkylfluorophosphoranes in improved yields and high purities,so that the products prepared are suitable for employment in batteryelectrolytes. Another object of the invention is to provide a processwhich avoids the formation of toxic and explosive by-products.

The object of the invention is achieved by electrochemical fluorinationof alkylphosphoranes or of alkylphosphanes of the general formula (II)with identical or different alkyl substituents on the phosphorus. Thispermits the synthesis of cyclic, linear and branchedperfluoroalkylphosphoranes of the general formula (I) from compounds ofthe general formula (II) in high yields by the following reaction scheme

where

n is 1, 2, 3, 4, 5, 6, 7 or 8,

m is +1 or −1,

X is H, Cl or F,

Y is 1, 2, or 3 and

Z is 3 or 5, with the condition that

X is H, Cl or F, if Z=3 and

X is Cl or F, if Z=5.

From the alkyls class use is made of cyclic, linear or branched methyl,ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl ligands.

The invention therefore provides a process for preparingperfluoroalkylfluorophosphoranes by an electrochemical synthesis.

The starting substances used according to the invention from thealkylphosphoranes and alkylphosphanes classes form the correspondingphosphonium salts in anhydrous hydrogen fluoride, and these have verygood solubility in hydrogen fluoride. Advantageously, solutions withphosphorane or phosphane concentrations from 30 to 60% are notcombustible, unlike the pure alkylphosphoranes or alkylphosphanes, andcan therefore be used as starting materials which are easy to handle.

It has been found that no explosive substances are formed during thenovel electrochemical fluorination of alkylphosphoranes oralkylphosphanes.

The main by-products of the novel electrochemical fluorination arephosphorus pentafluoride and fluoroalkanes, which in turn may be usedindustrially as ozone-friendly propellant, solvent or synthesis buildingblock.

In contrast to previous assumptions it has been found that it isparticularly the unsubstituted trialkylphosphanes having chain lengthsof C22 which have particularly high suitability as starting materialsfor electrochemical perfluorination reactions. Contrary to expectationstheir stability is very high. Whereas trimethylphosphane is extensivelydegraded by the electrochemical fluorination, probably due to thecleavage of difluorocarbene, etc., even triethylphosphane is convertedwith very good yields into triperfluoroethyldifluorophosphorane.

To prepare the compounds according to the invention, use is made of anelectrolysis cell which consists of, for example, a cylindricaldouble-walled vessel, and the material of which, e.g. stainless steel,is stable with respect to the prevailing reaction conditions. Theelectrolysis cell comprises an electrode package with alternating nickelanodes and cathodes made from HF-resistant materials with in each casean effective surface area of, for example 4.58 dm² for nickel anodes andnickel cathodes. The electrolysis cell has a commercially availablemeter to determine the consumption of current during the reaction. Tocarry out the process, the cell is cooled to temperatures of from −15°C. to 19° C., or temperatures up to 40° C. may be used with increasedpressure. Experiments have shown that good results are achieved at from−10° C. to 10° C. However, the temperature preferably used is 0° C.,since this temperature is particularly easily maintained, e.g. byice-water cooling. The cell has a reflux condenser to condense gaseousreaction products. The gas outflow is cooled to temperatures of from−10° C. to −35° C. Cooling to from −15° C. to −33° C. is preferred. Itis particularly preferable to carry out operations at −30° C. by usingethanol as cooling medium.

An appropriate amount of liquid hydrogen fluoride is pre-electrolysedfor from 2 to 100 hours, depending on moisture content. 48 hours aregenerally sufficient. The starting materials are used batchwise in theform of 10 to 70% solutions in HF, since these are not combustible. Theexperiments have shown that the best results are achieved with 30 to 45%solutions. The liquid reaction products are collected at the base of thecell. Gaseous products are conducted away via the reflux condenser andcondensed with the aid of two cooling traps in succession which arecooled to from −50° C. to −100° C. The temperature range used ispreferably from −60° C. to −85° C., since in this temperature range mostof the gaseous products condense. It is very particularly preferable towork at −78° C. with easy cooling by means of dry ice. The process iscarried out at a pressure of from 1 to 3 bar. Working at higher pressurerequires peripheral equipment specifically designed for this pressureand resulting in considerable costs. For cost-effectiveness reasons itis preferable to work at a small gauge pressure of from 1 to 0.5 bar.The reaction is particularly preferably carried out at atmosphericpressure (1 bar). The electrolysis takes place at a cell voltage of from4.0 to 6.5 V. The reaction is carried out at a current density of from0.1 to 3.5 A/dm². A current density of from 0.2 to 0.6 A/dm² isgenerally sufficient. Good results are achieved at a current density offrom 0.22 to 0.55 A/dm². To achieve virtually complete conversion of thestarting materials the electrolysis is terminated after from about 80 to200% of the theoretical electricity throughput. Good conversion isachieved at from 90 to 170% of the theoretical throughput. 95 to 150%throughput proved particularly suitable in the experiments. The liquidreaction product is periodically withdrawn and the volume withdrawn isreplaced by adding hydrogen fluoride with new starting material. Thetotal yield is given by the amount of reaction product from the reactionvessel and the reaction product isolated from the cold traps.

The reaction products may, immediately or after purification bydistillation, be converted into the corresponding phosphate usinglithium fluoride.

The process, which can be carried out at low cost and using simplematerials and apparatus, gives, in good yields, products of a qualitysuitable for use in batteries. This process does not form any explosiveor toxic by-products. The by-products do not destroy ozone and can beused as CFHC-substitute propellant gases.

The examples below are intended to describe the invention in greaterdetail but not to restrict the same.

EXAMPLES Example 1

Tris(pentafluoroethyl)difluorophosphorane (III)

The compound tris(pentafluoroethyl)difluoro-phosphorane was prepared byelectrochemical fluorination of triethylphosphane. A cylindricaldouble-walled vessel made from stainless steel with a total volume of360 cm³ served as electrolysis cell. The electrolysis cell has anelectrode package of alternating nickel anodes and nickel cathodes within each case an effective surface of 4.58 dm². The cell was cooled to 0°C. and had a reflux condenser with gas outflow at −30° C.

310 g of liquid hydrogen fluoride were firstly pre-electrolysed in thecell, and then a total of 158.2 g of a 36% triethylphosphane solution inhydrogen fluoride was added in five portions, as shown in the followingtable.

Amount of triethylphosphane Duration of electrolysis solution [g] [Ah]27.8 0 30.4 81.8 31.9 168.0 31.1 256.4 37.0 371.3

Gaseous products were conducted away via the reflux condenser and passedthrough two fluoropolymer traps cooled to −78° C. The electrolysis tookplace at a cell voltage of from 4.0 to 5.1 Volt and at a current densityof from 0.44 to 0.55 A/dm², and was terminated after 517 Ah throughput(133.8% of the theoretically required amount). Of the amount ofelectricity used, about 7% was used for drying the hydrogen fluoride.The liquid reaction product, which collects at the base of the cell, wasperiodically withdrawn and the volume discharged replaced by adding 131g of hydrogen fluoride. A total of 146.1 g of a clear liquid wasisolated and shown by ¹⁹F and ³¹P-NMR spectra to be practically puretris(pentafluoroethyl)difluoro-phosphorane. A further 5 g of thiscompound could be isolated from the traps cooled to −78° C.

The total yield of tris(pentafluoroethyl)di-fluorophosphorane wastherefore 73.5%.

³¹P-NMR spectroscopic data corresponded to those given in the literature(V. J. Semenii et. al., Zh. Obshch. Khim. (Russ.) 1985, Vol. 55, 122716-2720).

³¹P NMR, ppm: (CD₃COCD₃ film with 85% H₃PO₄ as standard),

−47.95 tsep

J¹ _(P,F)=1003 Hz J² _(P,F)=122 Hz

¹⁹F NMR, ppm: (CD₃COCD₃ film with CCl₃F as standard),

−49.76 dm (2F, PF₂)

−82.27 t (9F, 3CF₃)

113.81 dt (6F, 3CF₂)

J¹ _(P,F)=1003 Hz

J² _(P,F)−122 Hz

J³ _(F,F)=12.5 Hz

J⁴ _(F,F)=9.5 Hz

Example 2

Tris(nonafluorobutyl)difluorophosphorane (IV)

Tris(nonafluorobutyl)difluorophosphorane was prepared by electrochemicalfluorination of tributylphosphine. In this case the electrolysis cellhad a volume of 1.5 liters and effective anode and cathode surface areasof in each case 15.6 dm². The cell temperature was 0° C. and thetemperature of the reflux condenser was −20° C.

1125 g of liquid hydrogen fluoride was preelectrolysed for 100 hours inthe cell and then 268.0 g of tributylphosphine in 34.8 or, respectively43.6% solution in hydrogen fluoride were added in 7 portions, as listedin the following table.

Amount of tributylphosphine Duration of electrolysis [g] [Ah] 41.8 038.0 291.3 38.0 623.8 35.1 930.6 41.8 1430.0 35.8 1939.0 37.5 2414.9

The electrolysis voltage was from 4.5 to 5.2 V (cell voltage), thecurrent density was 0.32 A/dm² and the total use was 2918.4 Ah (146.5%of theoretical). Of the entire amount of electricity used, about 10% wasused for drying the electrolyte. The liquid electrolysis products whichseparated from the hydrogen fluoride solution were withdrawn in portionsfrom the base of the cell, and the volume was held constant by addingsupplementary hydrogen fluoride (total amount 1164 g). A total of 470 gof clear liquid was obtained as electrolysis product and ¹⁹F and ³¹Pspectra showed this to be practically puretris(nonafluorobutyl)-difluorophosphorane, corresponding to a yield of48.8%.

The NMR data correspond to the data known from the literature fortris(nonafluorobutyl)difluorophosphorane.

³¹P NMR, ppm: (CD₃COCD₃ film with 85% H₃PO₄ as standard),

−43.50 tsep

J¹ _(P,F)=1049.8 Hz J² _(P,F)=125 Hz

¹⁹F NMR, ppm: (CD₃COCD₃ film with CCl₃F as standard),

−46.97 dm (2F, PF₂)

−83.36 m (9F, 3CF₃)

109.43 dm (6F, 3CF₂)

121.88 m (6F, 3CF₂)

127.61 m (6F, 3CF₂)

J¹ _(P,F)=1049 Hz

J² _(P,F)=124.7 Hz

Example 3

Pentafluoroethyltetrafluorophosphorane (V)

49.0 g of dichloroethylphosphane in 63 g of hydrogen fluoride, i.e. 112g of 43.8% solution, were added during the electrolysis in 4 portions,as shown in the following table, to 308 g of pre-electrolysed liquidhydrogen fluoride in the electrolysis vessel of Example 1. The gaseousproducts were condensed in two polytetrafluoroethylene traps at −78° C.

Amount of dichloroethylphosphane Duration of electrolysis solution [g][Ah] 31.0 0 34.0 33.2 23.0 54.3 24.0 84.6

The electrolysis was carried out with from 4.5 to 5.4 Volt of cellvoltage and with a current density of from 0.22 to 0.44 A/dm² for 118.1Ah (98.2% of theoretical). 45 g of a solution which comprised about 15 gof pentafluoroethyltetrafluorophosphorane condensed in the cold trap.This corresponded to a yield of 17.7%.

The volatile reaction product was not isolated but converted into thecorresponding pentafluoroethylpentafluorophosphate product by adding2.25 g of lithium fluoride in HF solution. The NMR data correspond tothe data known from the literature forpentafluoroethyltetrafluorophosphorane.

³¹P NMR, ppm: (CD₃COCD₃ film with 85% H₃PO₄ as standard)

−40° C.

−54.37 ppm

Example 4

Tris(heptafluoropropyl)difluorophosphorane (VI)

The compound tris(heptafluoropropyl)difluorophosphorane was prepared byelectrochemical fluorination of tripropylphosphane. A cylindricaldouble-walled vessel made from stainless steel with a total volume of310 cm³ served as electrolysis cell. The electrolysis cell has anelectrode package of alternating nickel anodes and nickel cathodes within each case an effective surface of 3.75 dm². The cell was cooled to 0°C. and had a reflux condenser with gas outflow at −25° C.

230 g of liquid hydrogen fluoride were firstly pre-electrolysed in thecell, and then a total of 133.0 g of a 37.6% tripropylphosphane solutionin hydrogen fluoride was added in four portions, as shown in thefollowing table.

Amount of tripropylphosphane solution Duration of electrolysis [g] [Ah]33.0 0 31.0 91.8 32.0 189.8 37.0 282.3

The electrolysis took place at a cell voltage of from 4.0 to 5.1 Voltand at a current density of from 0.37 to 0.53 A/dm², and was terminatedafter 476.3 Ah throughput (129.4% of the theoretically required amount).Of the amount of electricity used, about 5% was used for drying thehydrogen fluoride. The liquid react on product, which collects at thebase of the cell, was periodically withdrawn and the volume dischargedreplaced by adding 135 g of hydrogen fluoride. A total of 95.6 g of aclear liquid was isolated and shown by ¹⁹F and ³¹P-NMR spectra to bepractically pure tris(heptafluoropropyl)difluorophosphorane. The yieldof tris(heptafluoropropyl)-difluorophosphorane was 53.2%. The ¹⁹F and³¹P NMR spectroscopic data correspond to those in the literature (V. J.Semenii et al., Zh. Obshch. Khim. (Russ.) 1985, Vol. 55, 12, 2716-2720):

³¹P, ppm: (CD₃COCD₃ film with 85% H₃PO₄ as standard),

−43.89 tsep

J¹ _(P,F)=1041 Hz

J² _(P,F)=123.9 Hz

¹⁹F NMR, ppm: (CD₃COCD₃ film with CCl₃F as standard),

−47.42 dm (2F, PF₂)

−82.49 m (9F, 3CF₃)

−110.40 dm (6F, 3CF₂)

−125.77 s (6F, 3CF₂)

J¹P,F=1040 Hz

J²P,F=124.6 Hz

J³F,F=14.0 Hz

Example 5

Tris(nonafluoroisobutyl)difluorophosphorane (VII)

The compound tris(nonafluoroisobutyl)difluorophosphorane was prepared byelectrochemical fluorination of tris(isobutyl)phosphane. In this case heelectrolysis cell had a volume of 1.5 liters and effective anode andcathode surfaces of in each case 15.6 dm². The cell was cooled to 0° C.and had a reflux condenser with gas outflow at −20° C.

³¹P NMR, ppm: (CD₃COCD₃ film with 85% H₃PO₄ as standard),

−41.35 tsep

J¹ _(P,F)=1086 Hz

J² _(P,F)=125.0 Hz

¹⁹F NMR, ppm: (CD₃COCD₃ film with CCl₃F as standard),

−45.98 dm (2F, PF₂)

−74.07 m (18F, 6CF₃)

99.20 dm (6F, 3CF₂)

180.49 m (3F, 3CF₂)

J¹ _(P,F)=1087 Hz

J² _(P,F)=124.9 Hz

1075 g of liquid hydrogen fluoride were firstlv pre-electrolysed in thecell, and then a total of 499.0 g of a 42.9% tri(isobutyl)phosphanesolution in hydrogen fluoride was added in five portions, as shown inthe following table.

Amount of tris(isobutyl)phosphane Duration of electrolysis solution [g][Ah] 104.0 0 96.0 315.2 102.0 699.9 99.0 983.6 98.0 1373.4

The electrolysis took place at a cell voltage of from 4.5 to 5.5 Voltand at a current density of from 0.20 to 0.35 A/dm², and was terminatedafter 2377.2 Ah throughput (149.5% of the theoretically requiredamount). Of the amount of electricity used, about 9% was used for dryingthe hydrogen fluoride. The liquid reaction product which separates fromthe hydrogen fluoride solution was periodically drawn off from the baseof the cell and the volume was held constant by adding supplementaryhydrogen fluoride (total amount 690 g). A total of 440 g of a clearfluid was isolated and was shown by ¹⁹F and ³¹P NMR spectra to be amixture of tris(nonafluoroisobutyl)difluorophosphorane,tris-(nonafluorobutyl)difluorophosphorane andnonafluorobutyl[bis(nonafluoroisobutyl)]difluorophosphorane. The yieldwas 57.2%. This mixture comprises about 10% of the isomers with one ortwo hydrogen atoms. Fractionated distillation in a Teflon apparatusallows tris(nonafluoroisobutyl)difluorophosphorane to be isolated as themain fraction.

What is claimed is:
 1. Process for preparingperfluoroalkylfluorophosphoranes of the general formula(C_(n)F_(2n+m))_(y)PF_(5−y)  (I) where n is 1, 2, 3, 4, 5, 6, 7 or 8 mis +1 or −1 and y is 1, 2 or 3, where the ligands (C_(n)F_(2n+m)) may beidentical or different, characterized in that the electrochemicalfluorination of alkylphosphoranes or of alkylphosphanes with identicalor different, linear, branched or cyclic alkyl substituents on thephosphorus is carried out in anhydrous HF at a cell temperature of from−15° C. to 40° C. at a pressure of from 1 to 3 bar, at a cell voltage offrom 4.0 to 6.5 V and at a current density of from 0.1 to 3.5 A/dm². 2.Process according to claim 1, characterized in that the fluorination iscarried out at from −10° C. to 10° C.
 3. Process according to claim 1,characterized in that the fluorination is carried out at 0° C. 4.Process according to claim 1, characterized in that the fluorination iscarried out at a pressure of from 1 to 0.5 bar.
 5. Process according toclaim 1, characterized in that the fluorination is carried out atatmospheric pressure.
 6. Process according claim 1, characterized inthat the reaction is carried out by a current density of from 0.2 to 3.5A/dm².
 7. Process according claim 1, characterized in that the reactionis carried out by a current density of from 0.22 to 0.55 A/dm².